In 1974, it was discovered that the Raman scattering signal of certain compounds could be enhanced by orders of magnitude proximate to metallic surfaces roughened on the scale of tens to hundreds of nanometers, a technique called Surface Enhanced Raman Spectroscopy (SERS).
When laser light scatters from a metal surface, typically one photon in a million interacts with the vibrational states of molecules adsorbed on the surface, and the frequency of the scattered photon is shifted accordingly. Averaged over time, the sum of the shifted photon frequencies (the Raman shift) is a vibrational spectrum of the adsorbed molecule. Because every molecule has its own unique fingerprint spectrum, in principle, the SERS response can identify of any chemical of interest.
A conventional SERS substrate prepared by electrochemical roughening yields a random pattern that happens to include some “hot spots” where surface plasmons resonate with the incident radiation—a phenomenon known as Surface Plasmon Resonance (SPR). Analyte molecules in this enhanced electromagnetic field are subjected to stronger polarizing effects and thereby support Raman scattering with higher efficiency. Experiments demonstrating SERS enhancement factors under conditions that were controlled to eliminate the possibility of direct contact between analyte and substrate have lent credence to this concept of electromagnetic enhancement, and based on those experiments and theoretical calculations the SPR is considered to account for the larger component of the SERS enhancement factor, typically of the order of 103 to 106.
A second effect, called chemical enhancement, has been suggested for molecules that become adsorbed onto the surface—thereby coupling the molecule's valence electron charge density with the substrate. This gives rise to extra bound energy states and presents a greater opportunity for energy coupling with the incident radiation. This mechanism is not as well established, but is supported by experiments demonstrating enhancement on the order of 101 to 103 from an analyte adsorbed onto an atomically smooth gold substrate.
Surface Enhanced Raman Spectroscopy (SERS) has the potential to detect minute quantities of organic compounds. The vibrational spectra obtained are unique fingerprints of chemical composition and bonding, thus providing excellent selectivity. SERS typically uses a rough solid surfaces as a substrate upon which molecules are adsorbed, either from solution or the vapor phase. However, laboratory substrate preparations are poor in terms of reproducibility, and typically the enhancement factors vary by orders of magnitude from point-to-point across a given substrate.
Thus there is a need for a process to make SERS substrates that exhibit high enhancement factors which are uniform across the surface, are stable when exposed to environmental conditions over long periods of time, and can be manufactured with a high degree of reproducibility. At present, SERS systems have seen little use in practical, field-portable chemical sensor systems, primarily due to poor reproducibility of SERS-active substrates.
Over the past 25 years, the general approach to preparing these surfaces has been empirical, with laboratory processes developed to produce surfaces for SERS spectra of analytes of interest. Historically, the first SERS-active substrates were electrochemically-roughened silver electrodes, which strongly enhanced the Raman spectrum of pyridine dissolved in water. Colloidal gold and silver particles suspended in a solvent containing certain analytes have also been used as SERS substrates.
However, conventional substrate preparation methods preparations are notoriously irreproducible. Thus, improved methods of manufacturing environmentally stable, sensitive, and reproducible SERS substrates are urgently needed.
U.S. Pat. No. 4,977,038 to Sieradzki et al. describes an electrochemical method of preparing micro- and nano-porous metallic structures. U.S. Pat. No. 6,203,925 to Attard et al. describes methods of preparing an ordered porous metal through an intermediate liquid crystalline phase. However, there is no suggestion in these patents to use metallic structures in any analytical technique.